I have an 02 WRX that is getting the swap done at Precision Tuning... Some of the stuff I read here doesn't seem right (at least for an 07 6mt to go into an 02). My mechanic told me that you can keep all the suspension things (struts, spring, etc.), the hubs, brakes, and rear dif as well.
As for the DCCD controller, that's definitely not optional, especially if you plan on using a dyno. The car will drive without it, but it isn't recommended in the long run since the transmission is looking for a signal. Without any signal, the torque split can vary in unpredictable ways and make your new tranny go boom.

I said complete, as in a complete swap, not the bare minimum. You can do an 07 6 speed into a WRX and just do the tranny and stubs and an auto driveshaft and thats it. If you do STI axles you have to do STI knuckles and if you do STI knuckles you have to do STI struts and springs. Trust me on that one, I've done this a few times now. If you do the STI rear diff you have to do STI axles and therefor STI knuckles and hubs and struts and springs. If you do STI knuckles you do STI backing plates which means Brembos. There are ways around all of that of, and you can mix and match a bunch of stuff, but this thread is to tell people how to do it the right way from the start.

And a DCCD controller is absolutely optional. With no signal your center diff goes wide open, as in no locking, which is fine. Torque split is a mechanical thing; your DCCD controller doesn't control torque split, it controls the amount of lock of the center diff. It's going to run just like any other open center diff. It won't run as well at a track and it won't run as well in crappy traction, but it won't hurt anything. IIRC the center diff has some mechanical locking in the 07, like 10% or something like that, even at wide open. It'll do some sweet burnouts too! Spiider has a model where you can turn off the DCCD for a reason. Keep on reading buddy.